This invention relates to a catalytic system for polyurethane propellants. More particularly, this invention relates to novel ferric catalyst for use in promoting the cure of polyurethane propellant compositions.
Prior to the present invention, a considerable research effort has evolved in an attempt to develop catalysts for promoting the cure of urethane based propellants.
Such propellants conventionally comprise a mixture of an inorganic oxidizer component, such as ammonium perchlorate, dispersed in a matrix of a polyurethane resin which acts as both a fuel and binder component. Other conventional additive materials, such as plasticizers, burning rate modifiers, anti-oxidants, wetting agents, anti-foaming agents, etc., may be employed if desired.
The curing of these propellant compositions generally takes place at temperatures ranging from about 110.degree. to 135.degree. F. Such high temperatures, however, often create problems of stress and strain in case bonded rocket motors. The use of lower cure temperatures in the range of 70.degree. to 80.degree. F would significantly reduce propellant bore strains and bond stresses in the case bonded rocket motors. A satisfactory balance between potlife and the time required for effecting a full cure is also a problem of curing polyurethane propellants at room temperature during which the accelerating effect of higher temperatures on the urethane reaction cannot be utilized. This is especially true in the case of lithium initiated, hydroxy terminated polybutadien (LiHTPB) prepolymers cures with a diisocyanate such as hexamethylene diisocyanate (HDI).
One catalyst which has been suggested for use in curing urethane propellants is ferric acetylacetonate [Fe (AA).sub.3 ]. At catalyst levels as low as 0.001 percent Fe(AA).sub.3, however, the propellant mix will be castable for only 30 minutes in case of HDI and somewhat longer for toluene diisocyanate (TDI) cured propellants but still requires 7-10 days for full cure. In the absence of a catalyst, the LiHTPB propellants are practically uncurable (weeks at 180.degree. F are required). Reduction of the catalyst level below 0.001 percent entails the danger of losing the catalyst in degradative reactions.
With the present invention, however, there has been found a number of ferric compounds that can be effectively employed as a catalyst for promoting the isocyanate-hydroxy curing reaction at room temperature under the conditions encountered in a solid propellant environment. In addition, mechanical processing characteristics, mechanical behavior and storage stability are not adversely affected.